Antagonists at muscarinic acetylcholine receptors (mAChRs) are commonly used in parkinsonian patients as an alternative to dopaminergic replacement therapies. The mAChR antagonists are very effective for treating symptoms, but their clinical utility is often limited by significant central and peripheral side effects. It is likely that the adverse effects of mAChR antagonists are due to the fact that available compounds are nonselective and have similar antagonist potencies at all of the five mAChR subtypes (termed Ml - M5). Development of an understanding of the individual mAChR subtypes involved in modulating basal ganglia function could provide a basis for development of mAChR antagonists that selectively block individual mAChR subtypes and may have antiparkinsonian efficacy with reduced adverse effect liability compared with the non-selective mAChR antagonists. We have made a major breakthrough in developing the first highly selective allosteric modulators of M1, M4, and M5 mAChR subtypes. In preliminary studies, we found that Ml, M4 and M5 mAChRs regulate neuronal excitability and inhibitory synaptic transmission in key nuclei of the basal ganglia. Specifically, the available data suggest to us that blockade of Ml and/or M4 mAChRs could have antiparkinsonian effects through reduction of pathologically increased and abnormally patterned neuronal activity in the subthalamic nucleus or the substantia nigra pars reticulata, while M5 activation may be beneficial, as M5 receptors appear to depolarize neurons in the substantia nigra pars compacta which may increase striatal dopamine levels. In the proposed studies, we plan to test these hypotheses by a thorough analysis of the electrophysiologic effects of our novel subtype-specific mAChR active compounds, and through in-vivo testing of these agents in acute and chronic rodent models of Parkinson's disease. Parallel immunohistochemcal studies (Core B) will examine the synaptic and subcellular distribution of the mAChR subtypes in the subthalamic nucleus and substantia nigra. These experiments are highly significant, as they may identify new classes of antiparkinsonian agents that promise effective treatment of parkinsonian patients with fewer side effects than currently possible.